The present invention relates to a transparent glass pane having a reflectance reducing coating and a combiner of a head-up display system. In particular, the glass pane is to be used as a vehicular window glass that is installed in an inclined position as in the case of a current automobile windshield, and the coating serves to reduce reflectance of visible light obliquely incident on the coated side of the glass pane.
There are various proposals of antireflection coatings for preventing or reducing the reflection of visible light from vehicular window glasses or windshields.
As a coating for the antireflection purpose, it is known to use a fluoride film and/or a laminate of a plurality of transparent oxide films which have different refractive indices. For example, JP 61-189501 A (1986) shows a three-layer coating made up of a first layer which is in contact with the glass surface and formed of Al.sub.2 O.sub.3 or CeF.sub.3 having a refractive index of 1.60-1.80, a second layer formed of a ZrO.sub.2 /TiO.sub.2 mixture having a refractive index of 1.95-2.15 and a third layer which is formed of MgF.sub.2 and has a refractive index of 1.30-1.45. JP 64-70701 A (1989) shows an antireflection three-layer coating which is electrically conductive and consists of a metal film coated on the glass surface as the first layer, a second layer which is a metal oxide film having a refractive index of 1.90-2.50 such as a TiO.sub.2 film and a third layer which has a refractive index of 1.35-1.50 and is formed of either SiO.sub.2 or MgF.sub.2. As a modified way of using a fluoride, JP 3-17601 A (1991) proposes to form an antireflection coating by applying a solution of a metal oxide precursor such as a silicon alkoxide to a glass surface, heating the sol film on the glass surface to convert it into a gel film and further heating the gel film in a fluorine containing atmosphere to thereby form a metal oxide film containing fluoride of the metal.
However, for vehicular window glasses, fluoride containing coatings are generally insufficient in durability, and the use of fluorine or a fluoride often offers inconvenience to the industrial manufacturing operations.
JP 4-357134 A (1992) shows a two-layer coating made up of a first oxide film which is in contact with the glass surface and has a thickness of 70-90 nm and a refractive index of 1.8-1.9 and an outer second oxide film which has a thickness of 110-130 nm and a refractive index of 1.4-1.5. JP 4-357135 A (1992) shows a three-layer coating made up of a first oxide film which is in contact with the glass surface and a thickness of 70-100 nm and a refractive index of 1.8-1.9, a second oxide film which has a thickness of 130-160 nm and a refractive index of 2.05-2.3, and an outer third oxide film which has a thickness of 110-130 nm and a refractive index of 1.4-1.5. The reflectance of the glass pane coated with the coating of either JP 4-357134 A (1992) or JP 4-357135 A ( 1992) becomes lower than that of the glass pane without the coating by 4.5-6.5% with respect to visible light obliquely incident on the coated side of the glass pane at an angle of 50-70 degrees with the normal.
In contrast with the above-mentioned antireflection coatings, a combiner of a head-up display system, which is formed on a front windshield, has a relatively high reflectance. As is known, a head-up display system allows the driver of a car to view a vehicular information of an instrument panel, while looking out a front windshield of the car, by projecting an image of the instrument panel onto a combiner. JP 64-43368 A (1989) discloses a method of partially coating a solid substrate with a thin film useful as a combiner by changing the liquid level of coating liquid of an incomplete container which is in tight contact with the substrate. JP (Utility Model) 4-131530 A (1992) discloses a combiner including a first reflection layer which is directly attached to a windshield and has a relatively uniform reflectance throughout visible light spectrum and a second reflection layer which is directly formed on the first layer and has a selective reflection with respect to incident visible light.
If a high reflectance film as a combiner is formed on a portion of a surface of the above-mentioned reflectance reducing coating, for example, by the method of either JP 64-43368 A (1989) or JP (Utility Model) 4-131530 A (1992) to provide a windshield with an antireflection coating and a combiner, optical design tends to become complicated. Furthermore, in some cases, the combiner tends to have a dark color and a high excitation purity due to interference with the reflectance reducing coating. With this, the visible light transmittance may be lowered, and the combiner may not have desired characteristics. Still furthermore, if at least five-layer films are formed in total on a glass pane to form a combiner on a reflectance reducing coating, the external appearance of the glass pane tends to be deteriorated and the strength of the films tends to be lowered.